I tried out both the Frontier .223 ammo and the 5.56 55 gr fmj and in my rifles the .223 ammo grouped significantly better than the 5.56 version. The 5.56 is probably loaded hotter, but I prefer accuracy, myself. Since the ammo is loaded at the Lake City plant, I would bet that it is LC brass with a Frontier headstamp. As for the bullets, they are Hornady's. If you reload, those 62 gr SPBT bullets are available in bulk from Graf's for a very good price with reasonable shipping.
https://www.grafs.com/retail/catalog/product/productId/77626
